[libcxx-commits] [libcxx] [libc++] Add tombstone traits to use in optional, variant (PR #98498)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,171 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___TYPE_TRAITS_DISENGAGED_TRAITS_H
+#define _LIBCPP___TYPE_TRAITS_DISENGAGED_TRAITS_H
+
+#include <__config>
+#include <__memory/construct_at.h>
+#include <__type_traits/datasizeof.h>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/is_constant_evaluated.h>
+#include <__type_traits/is_fundamental.h>
+#include <__type_traits/is_trivially_destructible.h>
+#include <__type_traits/void_t.h>
+#include <__utility/forward_like.h>
+#include <__utility/in_place.h>
+#include <__utility/piecewise_construct.h>
+#include <__utility/pointer_int_pair.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class>
+struct __tombstone_memory_layout;
+
+// bools have always exactly one bit set. If there is more than one set it's disengaged.
+template <>
+struct __tombstone_memory_layout<bool> {
+  static constexpr uint8_t __disengaged_value_    = 3;
+  static constexpr size_t __is_disengaged_offset_ = 0;
+};
+
+struct __tombstone_pointer_layout {
+  static constexpr uint8_t __disengaged_value_ = 1;
+#ifdef _LIBCPP_LITTLE_ENDIAN
+  static constexpr size_t __is_disengaged_offset_ = 0;
+#else
+  static constexpr size_t __is_disengaged_offset_ = sizeof(void*) - 1;
+#endif
+};
+
+// TODO: Look into
+// - filesystem::directory_iterator
+// - vector<T> with alignof(T) == 1
+
+template <class _Tp>
+struct __tombstone_memory_layout<__enable_specialization_if<is_fundamental_v<_Tp> && alignof(_Tp) >= 2, _Tp*>>
+    : __tombstone_pointer_layout {};
+
+template <class _Tp>
+struct __tombstone_memory_layout<_Tp**> : __tombstone_pointer_layout {};
+
+inline constexpr struct __init_engaged_t {
+} __init_engaged;
+inline constexpr struct __init_disengaged_t {
+} __init_disengaged;
+
+template <class _Tp, class _Payload>
+struct __tombstone_data {
+  using _TombstoneLayout = __tombstone_memory_layout<_Tp>;
+  using _IsDisengagedT   = remove_cv_t<decltype(_TombstoneLayout::__disengaged_value_)>;
+
+  _LIBCPP_NO_UNIQUE_ADDRESS _Payload __payload_;
+  char __padding_[_TombstoneLayout::__is_disengaged_offset_ - __datasizeof_v<_Payload>];
+  _IsDisengagedT __is_disengaged_;
+
+  template <class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr __tombstone_data(_Args&&... __args)
+      : __payload_(std::forward<_Args>(__args)...), __is_disengaged_(_TombstoneLayout::__disengaged_value_) {}
+};
+
+template <class _Tp, class _Payload>
+  requires(__tombstone_memory_layout<_Tp>::__is_disengaged_offset_ == 0)
+struct __tombstone_data<_Tp, _Payload> {
+  using _TombstoneLayout = __tombstone_memory_layout<_Tp>;
+  using _IsDisengagedT   = remove_cv_t<decltype(_TombstoneLayout::__disengaged_value_)>;
+
+  _IsDisengagedT __is_disengaged_;
+  _LIBCPP_NO_UNIQUE_ADDRESS _Payload __payload_;
+
+  template <class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr __tombstone_data(_Args&&... __args)
+      : __is_disengaged_(_TombstoneLayout::__disengaged_value_), __payload_(std::forward<_Args>(__args)...) {}
+};
+
+template <class _Tp, class _Payload>
+struct __tombstone_traits {
----------------
ldionne wrote:

```suggestion
struct __tombstoned_value {
```

?? Not super pretty but it explains what it does -- it adds a tombstone state to an object.

https://github.com/llvm/llvm-project/pull/98498